Millimeter wave imaging technology, particularly at frequencies from about 70-150 GHz, is actively being pursued for concealed weapons detection, all-weather landing aids, and imaging of building interiors. Passive imaging, where no active source is used (such as compared to radar technologies), has the advantage of not requiring a transmitter thus reducing the cost of the system. It relies on detection of the various levels of millimeter wave radiation naturally radiated by an object (that is its' emissivity) to differentiate between the object and its' background. Detection can be direct to a DC voltage which is proportional to the received integrated noise power, or else the received noise can be mixed down to a lower frequency and then detected. Direct detection has the advantage that it requires fewer parts, but the very small millimeter wave noise levels before detection generally require amplification (see L. Yujiri, “Passive Millimeter Wave Imaging,” IEEE MTT-S International Microwave Symposium Digest, 2006, pp. 98-101, June 2006). HRL Laboratories of Malibu, Calif. has developed a Sb-heterostructure diode that has been optimized to operate as a direct detector without bias voltage (see H. P. Moyer, R. L. Bowen, J. N. Schulman, D. H. Chow, S. Thomas, J. J. Lynch, and K. S. Holabird, “Sb-Heterstructure Low Noise W-Band Detector Diode Sensitivity Measurements,” IEEE MTT-S international Microwave Symposium Digest 2006, pp, 826-829, June 2006). Thus, direct detection without pre-amplification is possible (see J. Lynch, H. Moyer, J. Schulman, P. Lawyer, R. Bowen, J. Schaffner, D. Choudhury, J. Foschaar, and D. Chow, “Unamplified Direct Detection Sensor for Passive Millimeter Wave Imaging,” Proc. Of SPIE on Passive Millimeter-Wave Imaging Technology, eds. R. Appleby and D. Wilkner, Vol. 6211, 2006), which could enable a low-cost millimeter wave focal plane array if a suitable means for coupling an arrayable antenna to an array of the aforementioned Sb-heterostructure diodes could be devised. The present disclosure is directed to techniques for coupling an antenna, such as a horn antenna, to a diode without the need for intermediate pre-amplification.
FIGS. 1A-1C shows an initial effort at a solution to this problem. FIG. 1A shows a top the basic concept of a low-cost millimeter wave passive imaging array. Only two antennas are shown in this view for ease of illustration, but the array, which you typically be a two dimensional array, can be of any size desired. FIG. 1B is a side sectional view, the section being taken along line 1B-1B shown in FIG. 1A. In order to make the device shown in FIGS. 1A and 1B, diode chips 1 are mounted onto a printed circuit board 2 preferably using a flip-chip attachment process. The printed circuit board 2 has a conductive bottom surface 4a typically formed of a metal such as copper. The top surface of the printed circuit board 2 is patterned so that wiring 4c is formed by pattering the typical metallic surface of the printed circuit board 2. The wiring 4c on the top surface can be seen in FIG. 2A. Vias 4b conduct RF energy from the diode chip 1 and through the printed circuit board 2 to the bottom side thereof. A molded metal horn array 3 is soldered onto the topside wiring 4c on circuit board 2 preferably for efficient W-band image noise collection. FIG. 1C is a close up view of a diode chip 1, which has a pair of diodes 5a, 5b. The conductors 4d coupled respectively to diodes 5a and 5b pass each other without making electrical contact with each other in region 7 so as to make contact with the connectors 8 shown on opposite edges of chip 1. A thin layer of an insulator 6 allows the wiring from the diodes 5a, 5b to pass other each other with making connection. The connectors 8 can be bonded to the wiring 4c on the circuit board 2 using flip-chip bonding techniques known in the art.
While there are some common features between these initial efforts and the technology described subsequently herein, the present disclosure addresses some shortcomings of the this initial effort. In particular, the original diode chip 1 had RF pick-up antennas on the diode chip 1. It was subsequently discovered through electromagnetic simulation that the RF pick-up antennas needed to be on a printed circuit board substrate for wide band operation. Also, a back-short tuning cavity was fabricated using the printed circuit board itself, whereas in the present disclosure, an air-filled back-short cavity is explicitly made and used for increased operational bandwidth. The other major difference in these initial efforts is that the video output for a particular input polarization is single-ended, whereas in the present disclosure a differential output is described that can reduce interference on the DC lines, although for single linearly polarized field.
FIGS. 2A-2D shows a prior art (see J. Lynch, et. al., “Unamplified Direct Detection Sensor for Passive Millimeter Wave Imaging.” Proc. of SPIE on Passive Millimeter-Wave Imaging Technology, eds. R. Appleby and D. Wilkner, Vol. 6211, 2006) passive millimeter wave imaging transition that this invention improves upon. It can be seen in the plan view of FIG. 2A and the perspective view of FIG. 2B that the diode chip 1 is flip-chip mounted onto a fused silica substrate 2 that forms part of the back-short cavity. The video output 4 is taken off of the chip with a coplanar strips (ground-signal-ground) transmission line that is orthogonal to the RF pick-up antennas. The DC signal line is then bonded to a coaxial centerline pin/conductor. FIG. 2C shows a close view of the chip 1, while FIG. 2D is a bottom view of the chip 1 showing connections to conductors disposed on the substrate 2.
The new technology described in this disclosure integrates an RF choke into the RF pick-up probes (antennas) so that the DC lines can come directly off of the probe. This eliminates a lot of excess metal within the transition that causes parasitic reactance and DC/RF isolation in the DC lines. Also, the use of an air-filled back-short cavity of this disclosure rather than a fused silica filled cavity enables broader bandwidths to be achieved.